2003 monte carlo ss want crank,changed battery and had auternator checked 2 days ago . was fine. today wont crank again.nothing no lights or nuttin

Your ground is bad....must fix this by undo-ing the bolt that holds the end of the 1/0 black cable...the other end of what clamps to the battery...wirebrush the spot where it connects, bolt it back together...this way 300 cold cranking amps can get through~

Check your connections. Did the battery show charging when it did run? Will it start on a jump? Check the battery again to make sure it held it's charge or it may have a big power draw. In the 60s and 70s if the vehicle was jump started, the diodes in the alternator would short out. The wires to the alternator had to be disconnected when jump starting or the diodes would blow. I think it has been corrected, but if you get it started, check your charging system again.